1. Field of the Invention
The present invention relates to a charge and discharge control circuit configured to detect a voltage and an abnormality of a secondary battery, and to a battery device. More particularly, the present invention relates to a circuit configured to detect overcharge and overdischarge.
2. Description of the Related Art
A battery device is mainly used as a power supply for driving a portable electronic device (load). The battery device has an overcharge protection function of protecting a secondary battery from overcharge when a voltage of the battery to be connected to a load is increased due to charging to exceed an overcharge detection voltage (hereinafter referred to as VCU). Moreover, the battery device has an overdischarge protection function of protecting the secondary battery from overdischarge when the voltage of the battery is reduced due to discharging to fall below an overdischarge detection voltage (hereinafter referred to as VDL) (for example, see Japanese Patent Application Laid-open No. 2002-204532).
FIG. 3 is a circuit diagram for illustrating a related-art battery device. The related-art battery device includes secondary batteries 10 and 11 connected in series, a charge and discharge control circuit 100, a charge control FET 21, a discharge control FET 22, and load connection terminals 23 and 24. The charge and discharge control circuit 100 includes a control circuit 110, output circuits 111 and 112, comparators 113, 114, 115, and 116, reference voltage circuits 117 and 118, and resistor circuits 120, 121, 122, 123, 124, 125, 126, and 127. The comparators 113 and 114 detect overcharge. The comparators 115 and 116 detect overdischarge.
The related-art battery device operates as described below to control charge and discharge of the secondary battery.
A charge control FET gate connection terminal 104 outputs High to turn on the charge control FET 21 when a voltage of the secondary battery 10, namely, a voltage between voltage monitoring terminals 101 and 102 is less than VCU. Thus, the secondary batteries 10 and 11 are allowed to be charged. The charge control FET gate connection terminal 104 outputs Low to turn off the charge control FET 21 when the voltage between the voltage monitoring terminals 101 and 102 is equal to or more than VCU. Thus, the secondary batteries 10 and 11 are inhibited from being charged and are protected from the overcharge. A voltage of the secondary battery 11, namely, a voltage between voltage monitoring terminals 102 and 103 is similarly detected and controlled.
A discharge control FET gate connection terminal 105 outputs High to turn on the discharge control FET 22 when the voltage of the secondary battery 10, namely, the voltage between the voltage monitoring terminals 101 and 102 is more than VDL. Thus, the secondary batteries 10 and 11 are allowed to be discharged so that a load connected between the load connection terminals 23 and 24 can be driven. The discharge control FET gate connection terminal 105 outputs Low to turn off the discharge control FET 22 when the voltage of the secondary battery 10, namely, the voltage between the voltage monitoring terminals 101 and 102 is equal to or less than VDL. Thus, the secondary batteries 10 and 11 are inhibited from being discharged and are protected from the overdischarge. The voltage of the secondary battery 11, namely, the voltage between the voltage monitoring terminals 102 and 103 is similarly detected and controlled.
However, the above-mentioned charge and discharge control circuit has a problem in that, when a cell is added, a reference voltage circuit, a comparator and resistor circuit for detecting overcharge, and a comparator and resistor circuit for detecting overdischarge are added, and hence an area of the circuit is increased as the number of cells is increased.